A DFT Study of the Acidity of Ultrastable Y Zeolite: Where Is the Brønsted/Lewis Acid Synergism?

The effect of extra-framework aluminum (EFAL) species on the acidity of a HT6 cluster (T=Si, Al) was studied by density functional calculations. A higher acid strength, compared to an isolated site, was only found with Al(OH)2+. The increase in acid strength was not due to Brønsted/Lewis acid synergism, as previously reported, but to the stabilization of the conjugated base through hydrogen bonding with the EFAL hydroxy groups (see picture).